The present invention is generally directed to methods for reducing the distortion or dimensional variation in screw forms after heat treating and to screw forms having the reduced distortion or dimensional variations, more particularly to methods for hardening selected surface areas of screw forms and to screw forms hardened by the methods and, most particularly, to methods for hardening the contact zones of screw forms for use as rotors in positive displacement rotary axial screw pumps using a laser source and an optic and to rotors hardened by a laser source and an optic.
A standard screw pump operating under ideal conditions of pressure, fluid viscosity, and rotating speeds does not require hardened rotors to operate satisfactorily. When the pump is operated with thin fluids, at low speeds, at high pressures, or with contaminated fluids, hardening of the screw forms negates some adverse effects that these conditions can cause. Because prior standard processes for hardening screw forms required high temperatures up to about 1075.degree. F. to the entire mass of the screw forms, these prior processes sometimes caused unacceptable distortion of the screw form. In one specific prior standard process for hardening screw forms, the screw forms were first subjected to an air bath at about 750.degree. F.; then subjected to a first liquid bath at about 1075.degree. F.; then from the first liquid bath to a second liquid bath at about 750.degree. F.; then to a third liquid bath at room temperature with the third liquid bath at room temperature being followed by a fourth liquid bath at about 750.degree. F. which was, in turn, followed by a final liquid bath at room temperature. Each of these temperature changes represents a potential dimensional stability problem in the finalized screw form. In this application, the term "distortion" is defined as a change in physical dimension of a screw form or other body that cannot be controlled or predicted.
Various methods are presently used to harden screw forms and include gas nitriding, induction hardening, carbonitriding and liquid nitriding, for example, being some of the more common methods. The gas nitriding and induction hardening methods required machining of the screw form after the completion of the hardening process. To minimize machining operations, liquid nitriding was used after all machining processes have been completed.
Recently a manufacturer of screw forms spent considerable time and effort to ensure that the quality of the power rotors produced for rotary screw pumps were of the highest quality and accuracy achievable. Most of these efforts were focused on medium or elementary dimensional variations due to changes in rotor lead (lead being defined as the measure of length that it takes one thread to complete one revolution, or 360.degree., around a part) and run out (run out being defined as a composite tolerance used to control the functional relationship of one or more features of a part to a datum axis) issues. Run-out is a standard term used to describe the amount that a surface varies from being perfectly circular about a particular axis. While variations in lead and increased run-out that can be induced due to several variables in several processes were eliminated or at least controlled, the one variable that was not reduced or eliminated was the distortion effect of the heat treating process on the screw forms. During the screw form manufacturing process, the hardening process had been considered a distortion free process, but differences in material from lot to lot, and from location on each bar (center to end) were found to react differently to the liquid nitride hardening process. Screw form distortion was mainly seen in changes to the lead of the threaded section and run-out in various diameters of screw forms. The presence or absence of distortion in the hardened screw forms has been found to be a key to screw pump performance and consistency. To meet the competitive requirements of screw pump manufacturing, screw form distortion must be eliminated or at least reduced and controlled during the screw form manufacturing process.
Because of the geometry of the screw form or rotor thread, it was found to be impossible to accurately predict the end result of the liquid nitriding process, but experience showed unacceptable variation or distortion in the final part. This unacceptable variation was found in leads, run-out, and certain physical dimensions of the final hardened screw forms.
Through the years some work was done to determine the sizes and leads to manufacture on various rotor sizes to account for the changes that occur during the liquid nitriding hardening process. This has proven to be a never ending task as each lot of material responds slightly differently and is, thus, not consistent or predictable. The known method of manufacturing parts to one dimension to achieve a different dimension after being heat treated for compensation for sizes and leads has had no measurable effect on the run-out of the screw form or rotor after the liquid nitriding process was completed. Because run-out variation is not a growth but a distortion, it has not been predicted or controlled. It was believed the major cause of run-out distortion was the rapid temperature changes the screw form or rotor must go through during the liquid nitride hardening process. These temperature changes occur to the entire mass of the rotor and, thus, affect the entire shape of the rotor. As mentioned above, the liquid nitriding hardening method had previously been considered to be a distortion free hardening method for screw forms, such as, for example, rotors. However, because of the exacting tolerances required to produce screw forms to tight specifications for use with, for example, low noise screw pumps, the small amount of distortion that normally occurred during the liquid nitriding hardening process caused the screw forms to be uncontrollably and unpredictably out of the specified tolerances.
It is known that variation in screw form geometry or dimensions can be induced by the heat treating process and can cause distortion or dimensional variation in screw forms and, as a result, has adversely imparted the performance of screw pumps having the screw form therein in several areas. For screw pump rotors, these distortions or dimensional variations have been known to cause excessive backlash or interference in the mechanical system, excessive noise, shortened system life expectancy and higher system mechanical loss. When a screw form having out of tolerance dimensional variation is used in a screw pump, screw pump performance in the area of flow rate, power requirements, pump life, and noise level deteriorates.
Wide variations in these areas of screw pump performance have led to the requirement that screw pumps be rated to encompass a wider range of performance. This typically means that all screw pump ratings are very conservative when compared to the average screw pump for a specific application. As customers have become more aware of the need for optimizing screw pump performance to meet their system application requirements such as, for example, elevator applications, it has become important that the screw pump ratings become more accurate and approach a mean performance that meets the specific customer application specifications.
In the past, programs to eliminate as much manufacturing variation as possible in screw pumps were initiated. Some improvements were made in the design of the screw pump itself. Drawings were changed to a "Functional" method to minimize tolerance stack-up without tightening tolerances. Design modifications were made to reduce the number of variables that affect noise performance. Areas of manufacturing were also evaluated to determine specific operations that cause variation in screw pump parts. Because of these evaluations, the liquid nitriding hardening process was identified as the largest contributor to dimensional variation of the power rotor and as a major contributor to screw pump performance variation.
Thus, one key for reducing screw pump performance variation was to eliminate the pump to pump dimensional variation in the functioning elements of the screw pump. As used in this application, functioning elements are defined as those components that influence screw pump performance by directly impacting the fluid flow pattern and mechanical dynamics of the screw pump. By reducing the dimensional variation in the functioning elements, screw pump performance can be optimized and rated performance values more precisely defined.
One obvious solution might be to eliminate the heat treating process for screw forms. However, certain applications require hardened screw form rotors because of prior adverse experiences with soft screw form rotors. For this reason, a method that provides a hardened surface equal to or better than the current liquid nitriding process is required. This method would need to provide the hardness required while eliminating or at least significantly reducing, the degree of uncontrollable or unpredictable screw form distortion produced in the screw form rotor during the liquid nitride hardening process.
Another possible solution was to utilize a laser beam to heat treat the screw form surface. Numerous articles and technical reports have been written identifying laser heat treating as a possible method for hardening specific surfaces. The laser heat treating process was believed to be stable because of the localized temperature gradient. Since the entire mass of the screw form is not heated by the laser, the other material around the heat treated area helped stabilize the geometry or dimensions of the screw form as well as acting as a heat sink to control the rate of the temperature changes within the screw form. However, no laser heat treating process or any other process is known to have definitively demonstrated the capability for controlling screw form distortion and to sufficiently harden the power rotor. Specifically, no heat treated, hardened screw form, such as, for example, a power rotor, had been known to acceptably minimize screw form distortion and lead variations and to pass a 2,000 hour pump endurance test at about 900 psi.
Thus, there is a need for methods for reducing distortion or dimensional variations in screw forms and for screw-forms produced thereby, such as, for example, rotors used in screw pumps, which overcome the deficiencies listed above. Specifically, such methods should provide for screw form hardened contact zones; should significantly reduce distortion in the screw forms after the screw form hardening process; should improve the performance of screw pumps using the hardened screw forms; should provide for more precise, narrow performance rating of the screw pumps having the hardened screw forms; should decrease the manufacturing cost for hardening the screw forms; should reduce the noise level for screw pumps using the hardened screw forms; and should provide for improved operational life of screw pumps using the hardened screw forms.